The present invention relates to a focus detecting device of a photographic camera for detecting focusing conditions of an objective lens with respect to a photographic object, and more particularly to a focus detecting device including a device for projecting auxiliary light onto a low brightness object in aid of a focus detecting operation.
A typical example of such focus detecting systems is incorporated into a single lens reflex camera as a phase difference type focus detecting device. This system comprises a focus detecting optical system and a light measuring device. The focus detecting optical system splits a light reflected by a photographic object through the objective lens into two light bundles for impingement respectively upon two arrays of light receiving elements disposed in a position corresponding to a film plane. The light measuring device derives a deviation of the objective lens from an in-focus position with respect to the object from a phase difference between respective output signals of the arrays of light receiving elements.
Thus, the focus detecting device of the phase difference detection type relies for focus detection on the interrelation of the two output signals derived from the phase difference therebetween. Unless the photographic object has a certain degree of contrast, the interrelation of the signals will lose reliability or significance, thereby impairing a reliable focus detection. Moreover, when the brightness of the object is below a certain level, the object has an insufficient contrast for enabling the focus detection. In such a case, an auxiliary light projecting device is operated to project an auxiliary light beam onto the object to produce a sufficient contrast thereon.
Conventionally, the auxiliary light projecting device comprises a single optical system including a light emitting diode or the like to act as a light source for projecting a contrasty light image such as a striped pattern onto the object. The projecting region of this image, generally, is fixed to a region corresponding to a focus detecting area from which the focus detecting device receives the light beam. This enables the focus detecting device to carry out a focus detection as aided by the contrasty light image projected onto the object regardless of the position of the object.
The focus detecting device further comprises a control device operable in response to detected luminance of the object. This control device renders the auxiliary light projecting device inoperative when the luminance of the object is above a predetermined level for enabling the focus detection without the auxiliary light. When the brightness of the object is insufficient for enabling accurate focus detection under the ambient light condition, the control device causes the auxiliary light projecting device to project the auxiliary light onto the object.
The known focus detecting device of a camera, however, has the following disadvantage.
With the focus detecting device of the phase difference detection type, as noted above, an accurate focus detecting operation cannot be expected unless the photographic object has a certain degree of contrast. The object lacks in contrast not only when the luminance thereof is below a certain level but when the object itself, though its luminance may be sufficient, intrinsically has no contrast such as a monochromatic wall or the like.
According to the known focus detecting device, however, the auxiliary light projecting device is actuated to project auxiliary light only when the luminance of the object is at such a low level that an accurate focus detection is difficult or impossible under the ambient light condition. Although the focus detection is carried out as aided by the auxiliary light when the luminance of the object is at a low level, no focus detection is possible with the known system in the absence of contrast because of the nature or shape of the object itself even when the object has a high luminance level.